Microwave heating apparatus

ABSTRACT

A microwave heating unit for curing wire coatings comprising a microwave generator, a waveguide coupled to said generator and means for coupling the coated wire to said waveguide whereby said wire becomes the center conductor of a coaxial line. The outer conductor of the coaxial line is preferably circular in cross section. Propagation of microwave energy between the inner and outer conductors of the coaxial line necessarily passes through the wire coating, thereby curing said coating.

United States Patent Forster [54] MICROWAVE HEATING APPARATUS [72]inventor: Fxic 0. Forster, Scotch Plains, NJ.

[73] Assignee: Fsso Research And Engineering Company 22 Filed: May 27,mo

[21] Appl.No.: 40,911

Related Us. Application 11111.

[63] Continuation-impart of Ser. No. 684,139, Nov. 20,

1967, Pat. No. 3,551,199.

52 u.s.c1. ....219/10.ss,117/227 [51] 1111. C1. H05b9/06 [5a]misuse-r611 ..219/10.55

[56] Relerences Cited UNI'I'EDSTATESPATENTS 3,457,385 7/1969 Cumming..219/10.61 3,461,261 8/1969 Lewisetal... .....219/1o.55 3,535,48210/1970 Kluck... ..219/10.55 8/1962 Schmidt ..219/10.55

[ 1 Feb. 15, 1972 3,478,188 11/1969 White ..219/10.55 2,640,142 5/1953Kinn ..219/10.61 X 3,551,199 12/1970 Forster ..219/l0.55 X

FOREIGN PATENTS OR APPLICATIONS 1,452,124 8/1966 France ..2l9/l0.6l

Primary Examiner-R. F. Staubly Assistant Examiner-Hugh D. .laegerAttorney-Chasm and Sinnock and Anthony Lagani, Jr.

[ ABSTRACT A microwave heating unit for curing wire coatings comprisinga microwave generator, a waveguide coupled to said generator and meansfor coupling the coated wire to said waveguide whereby said wire becomesthe center conductor of a coaxial line.

The outer conductor of the coaxial line is preferably circular in crosssection. Propagation of microwave energy between the inner and outerconductors of the coaxial line necessarily passes through the wirecoating, thereby curing said coating.

7 Chins, 4 Drawing Figures PATENTEDFEB 15 I972 $543,054

sum 1 BF 2 ,VIS

FIG. JIo

5.0. Fqrs/er INVENTOR (lap/A7143 BY PATENT ATTORNEY PATENTEDFEB 15 I972643 O54 SHEET 2 [IF 2 FIG. JIb

FIG. 110

5.0. Forster INVENTOR i I,

BY PATEN ATTORNEY MICROWAVE HEATING APPARATUS CROSS-REFERENCE TO RELATEDAPPLICATION This application is a continuation-in-part of copendingapplication Ser. No. 684,139, filed Nov. 20, 1964, now U.S. Pat. No.3,551,199.

BACKGROUND OF THE INVENTION t Wire is coated with electrical insulationby various'means. Typically a thermoplastic such as polyethylene may beextruded onto a wire using a cross head extrusion die. Alternately, avulcanizable rubber suchas Ethylene 'PropyleneDiene Monomer (EPDM) maybe blended with curatives and cross head extruded over the wire to beinsulated. The extrusion Process may be Performed ate-temperatureinsufficient to acto cure vulcanizable polymers. For example,ferromagnetic or electrically conductive particles of less than 100microns in size are blended into a synthetic rubber and curedby'induction heating at a frequency of about 1 MI-I2. Induction heating,7

as the name implies, operates by inducing eddy currents, utilizeselectromagnetic coupling, in the ferromagnetic or electricallyconductive particles thereby heating the particles. The

surrounding rubber matrix is heated and cured by conduction from theparticles, e.g., see U.S. PatJNo. 3,249,658.

- Dielectric heating has been used to heat nonconductors having polarmolecules. For example, polyvinyl chloridemay bepressed into moldingpreforms" and preheatedby dielectric heating prior to introduction intoa compression mold. This heating technique relies on .the high polarityof the molecule to induce a heating: effect. The material to be'heatedis.placed between two plates which form a capacitance inan electroniccircuit. Thepol'arity of the, plates israpidlyreversed at a'frequency inthe range of about 1 to about 300 MHz. Heat is causedby the rapidvibration of the polar molecules attempting to align themselves with theconstantly changing field.

The degree of heating is relatedzto the die'lectric loss of-thematerial, that is, the energy dissipated :Ilt the dielectric. Ingeneral, the higher the frequency at which the dielectric heating isaccomplished, the greater the lossiness of the material andconsequently, the more efficient the energy conversion to heat.Additionally at the higher frequencies, the requirement for shielding isreduced. 1

These advantages: to very 'high frequency dielectric heating havestimulated much research in the use of a technique termed microwaveheating. Microwave heating is based on the principle thatelectromagnetic waves interact with a dielectric material, some of theenergy associated with these waves being stored and some beingdissipated. The-heatingeffect is a result of the dissipated energy(dielectric loss). The dielectric loss is caused by the frictional dragassociated with permanent or induced dipole orientation in thealternating electric field.

The term microwave heating" as used throughout the specification andclaims means. heating with electromagnetic radiation at about'800to30,000 MI-Iz; preferably'900 .to about 8,600 MHz; most-preferably atabout 915 to about 2,450 MHz. At these'frequencies, it is no longernecessary to confine the material to be heated between'plates of acapacitor. The electromagnetic radiation may be conducted, much like anyother fluid, by means of waveguides to the heating zone.

Though all polymer' molecules exhibit some polarity, with fewexceptions, however, the synthetic elastomers are essentially nonpolarand hence have a low. dielectric loss atthe lower frequencies. In themicrowave range it ispossibl'e to accomplish some heating due toincreased lossiness at the higher frequency. "Thus, it is possible tocure natural or synthetic rubber as itleaves an extruderhead by passingit through a;

microwave oven. The material is partially cured by being passed throughthe center of a helical metal waveguide which is connected to amicrowave generator running at 300 to 30,000 MHz. Curing is-completed bypassing the material through a conventional heater, e.g., see BritishPat. No. 1,065,971.

In order to effect a complete cure of such essentially nonpolarsynthetic polymers by microwave heating, it is generally necessary touse large amounts of inert tillers such as carbon black, the actualheating being accomplished primarily by thermal conduction from thefillers which are readily heated by electromagnetic radiation. v

' So far as this inventor is aware, it has not been heretofore possibleto apply microwave heating to wire coating operations for severalreasons. The nonuniform size of the inert fillers results in nonuniformheating which causes hot spots and bum-out of the relatively thininsulation coating. Furthermore, thewire, being a' conductor, acts as anantenna and transmits energy along its length resulting in high energylosses. Thus, the energy available for heating is reduced toinsignificant levels.

SUMMARY OF INVENTION It has now been found that synthetic polymers canbe successfully cured in wire coating operations by filling the polymerwith finely divided metal particles and using coaxial line/waveguidecoupling techniques to heat the polymer to cure temperatureswith-microwave heating.

The filler material particle size is critical and must be less than 10microns.

, The wire to'be coated acts as the center conductor of the coaxialline. Toreduce energy losses along the center conductor, the coaxialline, which extends to either side of the waveguide, is equippedwithcavities or baffles which attenuate the losses to tolerable limits.

DETAILED DESCRIPTION Any vulcanizable, extrudable elastomer may be usedin the practice of this -invention. Typical of such vulcanizableelastomers are natural rubber, butyl rubber, halogenated butyl rubber,Ethylene Propylene Diene Monomer (EPDM) and Neoprene rubbers.

Saturated peroxide crosslinkable polymers such as polypropylene,polyethylene and ethylene propylene rubber may also be cured by thetechnique of this invention.

In addition to solid elastomers and polymers, plastisols may be used asthe coating material, for example, a plastisol of PVC having a curingtemperature of about 330 F. and a room temperature viscosity in theuncured state of 160,000 c.p.s.,

having suspended therein the metal fillers of this invention, may becoated on a wire and cured by microwave heating.

The expression butyl rubber as employed in the specification and claimsis intended to include copolymers made from a polymerization reactantmixture having therein about IO-99.5 percent by weight of an isoolefinwhich has about four to sevencarbon atoms and about 30-0.5 percent byweight ofa and a Wijs Iodine'No. of about 0.5t5-aboutl 50; preferably 1to 15. The preparation of butyl rubber is'descfibed3.in .U.S. Pat. No.2,356,128, which is incorporated herein by reference.

' Theterm EPDM is used in the sense of its definition as foundinASTMD-14l8-64 and is intended to mean a terpolymer containing ethyleneand propylene in thebackbone and a diene in the side chain. Illustrativemethods for'producing these terpolymers are found in US. Pat. No.3,280,082

and British Pat. No. 1,030,989; which are incorporated herein byreference.

Any EPDM may be used in the practice of this invention. The preferredpolymers contain about 50 to about 70 wt.

ethylene and about 2.0 to about 5 wt. of a diene monomer,

the balance of the polymer being propylene. Preferably, the polymercontains about 50 to about 60 wt. ethylene, e.g., 56 wt. and about 2.6to about 4.0 wt. diene monomer, e.g., 3.3 wt.

The'diene monomer is preferably a nonconjugated diene. Il-

' lustrative of these nonconjugated diene monomers which may be used inthe terpolymer (EPDM) are hexadiene, dicyclopentadiene, ethylidenenorbornene, methylene norbomene, propylidene norbornene and methyltetrahydroindene. The

particle diene used does not form a critical part of this in'vention andany EPDM fitting the above description may be used. Atypical EPDM isVistalon 4,504 (Enjay Chemical Comparty.) a polymer having a Mooneyviscosity at 212 F. of about 40, prepared from a monomer blend having anethylene content of about 56 wt. and a nonconjugated diene content ofabout 3.3 wt.

Neoprene rubbers are described in a text entitled The Neoprenes byMurray and Thompson; DuPont, March 1963. There are two general types ofNeoprenes, G-type and W- type. The G-types differfrom the W-type in thatthe former are interpolymerized with sulfur and containthiuram'disulfide, whereas the W-type Neoprenes contain no elementalsulfur, thiuram disulfide or other compound capable of decomposing toyield either free sulfur or a vulcanization accelerator.

The term plastisol as used in this specification means a dispersion offinely divided resin in a plasticizer. When the plastisol is heated, theplasticizer solvates the resin particles, and the mass gels. Withcontinued application of heat, the mass fuses to become a conventionalthermoplastic material. The preparation of plastisols is well known tothe artand will not be discussed in detail.

Illustrative of a plastisol suitable for use in this invention is 100parts by weight of a dispersion grade polyvinyl chloride dispersed in 65parts by weight based on the PVC of a plasticizer such as diethylhexylphthalate.

Though this invention is directed toward the curing of coatings whichhave been extruded on a wire, it is not in tended to be limited solelyto that method of coating application. a

The term curable as used in the specification and claims is intended tocover the range of materials described above whichmaybe vulcanized,cross-linked by peroxide cures or cured in the sense of converting aplastisol to a thermoplastic.

It will be readily evident to one skilled in the art that ex pandablepolymer formulations may be used in the practice of this invention. Thepreparation of expandable preparations is Decomposition temperature ofavailable blowing agents is known to those skilled in theart; hence, thepreparation of these compositions will not be discussed in detail.

In addition to being used to cure polymers coated on electricalconductors, the process disclosed herein is suitable for use in dryingpaper wrapped conductors. In some electrical products, e.g.,transformers, the wire is wrapped with paper or other celluloseinsulators from which moisture must be removed. The apparatus and methodof this invention may be used to remove that moisture.

Illustrative of the metals suitable for use in the practice of thisinvention are iron, aluminum, copper, nickel, tin, zinc,

and alloys of nickel w'ith cobalt. In principle, any metal may be used.Such metals asthenoble metals, however, are excluded purely by economicconsiderations though' they would form operable embodiments of thisinvention.

Metals such as tin or zinc may be reduced to the proper particle size bymechanical grinding in methanol at 40 C. (Heusler technique). l

The other metals, iron, for example, may be reduced to the properparticle size by preparing alloys with other elements which areremovable by water, weak acids or alkali. Examples of such otherelements are aluminum, silicon, sodium, calcium or magnesium.

For example, an alloy of nickel and aluminum containing about 50 toabout 70 percent aluminum is prepared, annealed at about 800l,000 C. forseveral hours, and quenched to prevent a phase separation of the twometals. The alloy is then converted to a powder (ca. 200-325 mesh). Thealuminum is dissolved in alkali; usually a 20 percent water solution ofNaOH, the powder reacting readily with the cold solution with liberationof hydrogen. The heat generated by the reaction brings the liquid to aboil. The water, escaping as'steam, is replaced in order to maintain thevolume of solution. Finally, the mass is digested for several hours at ll 8-] 20 C.

The nickel sludge in the tank is washed free of lye with cold water. Thefinely divided nickel is then dried by adding an organic solvent or oil,heating and agitating until the drying is completejlf desired, the oilmay be a process oil which would ordinarily be used in compounding theelastomer to be coated onto the wire.

In the practice of this invention, the metal particle size is ofcritical importance and must be less than 10 microns, i.e., 0.5 to about10 microns.

; It is quite surprising that the products of the present invention arenonconductors of electricity. It has been foundthat withirilimits, thedielectric constant of the metal-filled polymer increases withincreasing metal filler particle size and yet the prior art productswhich make use of large particle size metal fillers are excellentelectrical conductors; indeed, the prior art'products havebeen suggestedfor use as the plates of capacitors and not as the dielectric mediumbetween the plates, see Metal-filled Plastics, page 195, by JohnDelmonte, Reinhold Publishing .Corp. (New York, 196i Furthermore, itwould be expected that the electrical conductivity of the metal-filledpolymers would rise sharply with in-' creased loadings of metalparticles, elg., see U.S. Pat. No. 3,21 1,584, issued Oct. 13, 1965, toJ. E. Ehrreich. However, it has been unexpectedly found that theelectrical conductivity of the present metal-filled polymers risesimperceptibly in contrast to the almost asymptotic rise in thedielectric constant at high levels of metal loadings.

If the metal particles are substantially greater than microns, therewill be metal to metal contact and the elastomer filled'with the metalwill be conductive and useless as a wire insulation. Between about 20 to100 microns the metal particles 'are sufficiently large to act asantennae. Hence, they will reirradiate substantial amounts of theelectromagnetic energy absorbed, thereby reducing the heating efficiencyto a very low level. Between about 10 to about 20 microns the metalparticles do not act as antennae but their area to mass ratio is suchthat heat transfer to the surrounding elastomer matrix is slow. Hence,overheating of the metal particles results with subsequent hot spots andburn-out of insulation. Below 10 microns, on the other hand, the area tomass ratio is such that heat transfer to the surrounding matrix is rapidenough to result in uniform heating of the elastomer without localizedoverheating in the vicinity of the metal particles. Preferably, theparticles have a particle size range of 0.5 to about 10 microns with anaverage particle size of about 5 microns.

An additional advantage of the small particle size is that the metalacts as a reinforcing filler in much the same manner as inorganic orcarbon black reinforcing fillers.

In its more preferred embodiment the metal fillers of this inventionhave a particle size rangeof less than 1.5 microns. Preferably, theparticles are made up about one-third of particles in the 1-1.5 micronrange, one-third in the0.5'l micron range and one-third in the 0.5micron range.

Metal particles of such small particle size are notoriously pyrophoricand are normally handled under water or solvent. In order to utilizesuch conventional blending means as rubber mills, banking mixers, etc.,to incorporate the metal tillers into the polymers, it is necessary totreat the metal particles to eliminate pyrophoric tendencies.

It has been found that treating the metal particles with organosilanes,in particular vinyl silanes, results in a silane coating on the particlewhich allows it to be safely handled in air. Metal particles so treatedmay be readily blended into polymers on conventional equipment withoutany danger.

The term organosilane compound as employed herein includes silanes,silanols (the corresponding partially or completely hydrolyzed forms ofsilane), siloxanes (the corresponding condensation product of thesilanols) and mixtures thereof. The organosilane compound may berepresented by the formula:

wherein R is a C C, radical containing vinyl-type unsaturation selectedfrom the group consisting of alkenyl, styryl, alkenoylalkyl andalkenoyloxyalkyl; X is selected from the group consisting of hydroxyl,alkoxy and acyloxy, R and R are independently selected from the groupconsisting of hydroxyl, methyl, alkoxy, acryloxy and R Nonlimitinguseful compounds which may be employed are the following: vinyltri(beta-meth-oxyethoxy )-silane, vinyl triethoxy silane, divinyldiethoxy silane, allyl triacetoxy silane; and in place of the vinyl andallyl groups of the above-named compounds, the corresponding styryl,acryloalkyl, methacryloalkyl, acryloxy propyl and methacryloxy propylcompounds. All of the silanes are convertible into the usefulcorresponding silanols by partial or complete hydrolysis with water. Thepreferred organosilanes of choice are gamma-methacryloxypropyltrimethoxy silane and vinyl tri-(beta-methoxyethoxy)-silane.

The silanes are applied to the metal particles in solution. Afterpreparation of the metal particles by either Raney or Heuslertechniques, the particles are transferred to an organic solvent such asC -C normal alkanes, e.g., hexane heptane, C -C aromatics, e.g.,benzene, toluene, xylene, carbon tetrachloride, trichloromethane ormineral spirits. The silane is then added to the solution with constantstirring for about 1 to about 10 minutes. Preferably, about 0.01 toabout l wt. silane based on the metal is used; more preferably, about0.05 to about 0.5 wt. most preferably about 0.1 to about 0.2 wt. e.g.,0.1 wt. After silane treatment, the metal may be removed from thesolvent by filtration, centrifugation or other mechanical means andsafely handled as a powder. The organosilane attached itself to themetal surface, prevents reaction of the metal powder with air.

It has been found that at least 0.1 part of metal powder must be blendedinto 100 parts of elastomer in order to effectively cure the elastomerby microwave heating. Preferably 0.2 phr. to about 18 phr., based on theelastomer, of metal powder is blended into the elastomer; morepreferably about 0.5 phr. to about 5 phr.; most preferably about 0.5phr. to about 1.5 phr., e.g., 1 phr.

In addition to the metal powder, various curatives, compounding aids andextender oils may be incorporated into the elastomer. Any curative knownto the art may be used for the various wire coating materials suitablefor use in the practice of this invention.

For example, elastomers such as butyl rubber or EPDM may be sulfur curedby such curatives as heavy metal dialkyl dithiocarbamates and quinoidcompounds. Typically, in the vulcanizing of EPDM suitable sulfur curemay be obtained by the use of certain heavy metal dialkyldithiocarbamates in conjunction with a thiourea, a metal oxide andmercaptobenzothiazole as cure activators.

Typical of the metal oxide cure activators which may be used are ZnO,Pb0 and MgO. Preferably the metal oxide cure activator is used at about2.5 to about 10 phr., based on the rubber, more preferably about 4 toabout 6 phr., e.g., 5 phr.

The heavy metal thiocarbamates usable in this invention have the generalformula:

wherein R is an alkyl group having from one to four carbon atoms andpreferably one to two carbon atoms; R is an alkyl, aryl, alkaryl orcycloparaffin group having from one to 10 carbon atoms and is preferablyan alkyl group having one to four carbon atoms; x is the valence of theheavy metal and can be an integer of two to four; the heavy metal isselected from those elements in groups I-B, II-B, IV-A, VI-A and VIII ofthe Periodic Chart of the Elements as published on pgs. 56 and 57 of theHandbook of Chemistry by Lange, eighth edition, 1952.

The dithiocarbamate salt may be a single salt or a mixture of salts,e.g., zinc dimethyl dithiocarbamate (methyl zimate) may be combined withtellurium diethyl dithiocarbamate (Tellurac). Other dithiocarbamatesthat are suitable for the purposes of this invention include seleniumdiethyl thiocarbamate, lead dimethyl dithiocarbamate, tellurium benzyldithiocarbamate, zinc butyl dithiocarbamate, etc. For best results, thethiocarbamate portion of the blend should comprise either the zinc ortellurium salt alone or a combination of these salts.

The thiocarbamates are used in a range of about I to about 5 phr. basedon the EPDM, preferablyabout 2 to about 5 phr. and most preferably about3 to about 4 phr. Typically, mixtures of dithiocarbamates comprisingabout 0.5 to about 1.5 phr. of tellurium diethyl dithiocarbamate, e.g.,0.8 phr., may be used in conjunction with about 2 to about 4 phr., basedon the EPDM, e.g., 3 phr., of zinc dimethyl dithiocarbamate.

Illustrative of the thioureas which may be used in the practice of thisinvention are thiocarbanilide (A-l), 1,3-diethyl thiourea (Pennzone E),1,3-dibutyl thiourea (Pennzone B). Preferably, the thioureas are used inthe range of about 1 to about 5 phr. based on the EPDM, more preferablyabout 2 to about 5 phr., e.g., 3 phr.

Mercaptobenzothiazole is also used as a cure activator. Typically, themercaptobenzothiazole is used at about 0.5 to about 3 phr. based on theEPDM; preferably about 1 to about 2 phr., e.g., 1.5 phr.

Other additives which may advantageously be used in the practice of thisinvention are various conventional rubber processing aids andplasticizers such as paraffinic or naphthenic process oils,microcrystalline waxes, tributyl ethyl phosphate (KP methyl hydroxystearate (Paricin-l and vulcanized vegetable oil such as that producedby the reaction of soya oil with sulfur monochloride (Factice 57-5). Theterm microcrystalline wax" as used in this specification means petroleumderived waxes characterized by the fineness of their crystals indistinction to the larger crystals of paraffin wax.

The term quinoid compound" means any dinitroso compounds, dioximes andsimilarly related compounds having an ortho or para quinoid aromaticnucleus or compounds which can be converted into such structure.Illustrative of these quinoid compounds are poly-p-dinitrosobenzene(Polyac), N- methyl-N,4-dinitrosoaniline (Esastopar), N-(2-methyl-2-nitrosopropyl) 4-nitrosoaniline (Nitrol) and p-quinone dioxime (GMF).

The saturated synthetic polymers such as ethylene propylene rubber,polypropylene and polyethylene are peroxide curable. Suitable peroxidesare those well known in the art for cross-linking these materials suchas di-tert.-butyl peroxide, 2,5-dimethyl-2,5-bis-(tert.butyl peroxy)hexane, 2,5- dimethyl-2,5-bis-(tert.-butyl peroxy) hexyne-3, benzoylperoxide, di-tert.-butyl-diperphthalate, tert.-butyl per-,

benzoate, dicumyl peroxide, cumene hydroperoxide, 2,4-di- (tert.-butylperoxyisopropyl) benzene, tert.-butyl cumyl peroxide, etc. and mixturesthereof.

It is common practice to propagate electromagnetic radiation in awaveguide by means of coaxial line/waveguide coupling. The outerconductor of the coaxial line is connected to an outer wall of thewaveguide while the center conductor protrudes into and terminates inthe free space within the waveguide. Similarly, the coaxial line can actas a pickup for microwaves flowing through the waveguide. The waveguideis closed at the coupling end and the center conductor of the coaxialline is located about one-fourth wavelength of the microwaves beingtransmitted, from the closed end of the waveguide.

Within the coaxial line radiation is between the center conductor andthe outer conductor of the coaxial line in a radial direction. By thismanner, microwaves may be transmitted in muchthe same way a current istransmitted along an electrical conductor.

In the practice of this invention, a modified coaxial line/waveguidecoupling unit is utilized to heat the aforementioned vulcanizablepolymer. For example, the coaxial line may be extended to either side ofthe waveguide, the wire to be coated acting as the center conductor ofthe coaxial line. To avoid large power losses, it is necessary toattenuate or suppress the radiation traveling along the coaxial line.

Referring now to the drawing, in particular FIG. I, numeral 11designates a spool of electrical conducting wire, l2. The wire, 12, ispassed through a cross head extruder die, 13, wherein the wire, 12, iscoated with the curable composition of this invention,'said coated wire,14, being passed through a rectangular waveguide, 15, having an enclosedend, 16. Extending to either side of the waveguide, 15, and concentricwith the coated wire, 14, are outer conductors, l7 and 18, of a coaxialline. The coated wire acts as the center conductor of the coaxial line.A microwave generator, 19, is coupled with the waveguide, 15, through anisolator, 20, and variable attenuator, 21, and propagates a wave ofelectromagnetic radiation through the waveguide, 15, toward the coatedwire, 14.

The portion of the coated wire within the waveguide is heated directlyby the electromagnetic radiation which is also picked up by the centerconductor wire, 14, of the coaxial line. Radiation between the centerconductor, 14, and the outer conductors, l7 and 18, of the coaxial linecontinues to heat and cure the coating throughout the external (to thewaveguide) heating zones, 22 and 22, until said radiation is suppressedor attenuated by the baffles, 23, or resonating cavi-' ties, 24.

The baffles attenuate the electromagnetic radiation by creating'amismatch between the center and outer conductors of the coaxial line.The resonating cavities impede or suppress the propagation radiation byabsorption of energy. In practice, it is preferred to combine bothsuppression means by utilizing alternating cavities and baffles.

Referring to the drawing, in particular FIG. Ila, numeral 17 designatesthe outer conductor of the coaxial line. Connected to the conductor, 17,is a series of resonating cavities, 24. The width of the cavity, 25, isM4, i.e., one-quarter of the wavelength of the electromagneticradiation. The inner diameter of the cavity can be equal to or greaterthan that of ing. The diameter of the cavity, 24, is 2r where r=2C. The

distance between cavities, 28, is not critical.

Baffles are somewhat similar to the resonating cavities discussed above.Referring to the drawing, in particular FIG. Ilb, numeral 18 designatesthe outer conductor of a coaxial line. Connected to the conductor, 18,is a series of baffles, 23. The width of the baflle, 29, is M4, i.e.,one-quarter of the wavelength of the electromagnetic radiation. Wherethe resonating cavities, 24, provide a space within the system, thebaifles, 23, present a restriction and impede radiation in a mannersimilar to throttling the flow of fluid in a line. The baffles areconnected by sections of insulating material, 30, e.g., Teflon, thediameter of which, C, is the diameter of the finished wire. The bafflediameter is 2r, where r=2C.

In its preferred embodiment the radiation suppression means constitutesa series of alternating bafi'les, 23, and cavities, 24, as shown in FIG.llc. Transition sections, 31, the dimensions of which are not critical,are required between the resonating cavities and baffles.

It will be obvious to those skilled in the art that a single microwavegenerating waveguide may be adapted to operate more than one curingunit. In such a case, the waveguide is modified to a T' transitionsection, the original waveguide forming the root of the T. The outwardlyextending portions of the T are waveguides each of which is coupled to acoaxial line. The individual coaxial lines should be isolated from oneanother by conventional isolation means in order to avoid feedback fromone unit to the other. The waveguide may be divided in a similar mannerto operate a multiplicity of coaxial lines.

The following examples serve to illustrate the manner in which theprocess of this invention may be carried out and the benefits derivedtherefrom.

EXAMPLE 1 A butyl rubber composition having the formulation shown belowwas prepared using conventional blending techniques.

Component Parts by Weight Enjay Butyl O35 I00 Calcined clay l 10 Ironparticles (av. 5p.) 4

w o, a I 5 Paraffin wax 5 LM polyethylene 5 Quinone dioxime 1.5Mercaptobenzothiazole 4 (I) Butyl rubber having 0.8 mole unsaturationand a Mooney viscosity at 2 l 2 F. of about 4 1-49.

(2) A petroleum derived paraffin wax having a melting point of F.

(3) A low molecular weight polyethylene having a molecular weight ofabout 20,000 to 50,000. (Weight average).

This composition is extruded at about 200 F. through a cross headextruder die having a %-inch diameter bore onto a /;-inch diameter wire.The coated wire is passed through theunit described above wherein 2L(FIG. I) is 4 feet and C (FIG. II) is 0.275 inches.

The microwave generator operates at 2,450 MHZ. and 5 kw. microwavepower. The total curing time in the heating zone is about 20 seconds,i.e., line speed ca. 12 ft./min.

EXAMPLE 2 A l/l6-inch diameter wire is coated with the ethylenepropylene rubber composition shown below by passing the wire through across head extruding die having a Va-inch diameter bore, the EPR beingextruded at about 200 F.

Component Parts by Weigh! Enjay Vistalon 404") I00 Calcined clay 1 l0Aluminum particles (av. Sn) 5 Dicumyl peroxide 2.8

Triallyl cyanurate 1.5

(l) Ethylene propylene rubber having an ethylene content of 40-46 wt.and a Mooney viscosity at 2! 2 F. of about 35-45.

The microwave unit of Example. 1 is used except that 2L equals 2 ft., Cis 0.135 inches and the line speed is about 6 ft./sec., i.e., curingtime seconds.

The coated wires produced by the method of Examples 1 V and 2 areequivalent in physical and electrical characteristics to conventionallycoated wires.

Since it is readily evident that many different embodiments may be madewithout departing from the spirit of this invention, it is not intendedto limit the scope thereof to the particular embodiments disclosedherein.

What is claimed is:

l. A microwave heating unit suitable for curing the insulating coveringof an insulated electrical conductor which comprises:

a. a microwave generator operating at a frequency of about 800 to about30,000 Ml-lz.;

b. at least one waveguide, said waveguide having at least one'endenclosed and having two aligned diametrically opposed openings in saidwaveguide, said openings being sufficient to pass said insulatedconductor therethrough and being located a distance equal to one-quarterwavelength of the operating frequency of the microwave generator fromtheclosed end of said waveguide, said waveguide being coupled to saidmicrowave generator;

c. an outer conductor of a coaxial line concentric with said openings,communicating with and outwardly extending from the waveguide, saidouter conductor being spacially oriented in such a manner that itscentral axis is coincid'ent with the center of both of said opening;

d. a center conductor of said coaxial line concentric with said outerconductor, said center conductor comprising an electrical conductorhaving a curable coating; and

e; means for suppressing microwave radiation transmitted,

via the waveguide, to a center conductor of the coaxial,

comprises at least three resonating cavities in series.

- 7. The apparatus of claim 1 wherein the suppressing means Q is analternating combination of baffles and resonating cavities comprising atleast one bafi'le and at least two resonating cavities.

1. A microwave heating unit suitable for curing the insulating coveringof an insulated electrical conductor which comprises: a. a microwavegenerator operating at a frequency of about 800 to about 30,000 MHz.; b.at least one waveguide, said waveguide having at least one end enclosedand having two aligned diametrically opposed openings in said waveguide,said openings being sufficient to pass said insulated conductortherethrough and being located a distance equal to one-quarterwavelength of the operating frequency of the microwave generator fromthe closed end of said waveguide, said waveguide being coupled to saidmicrowave generator; c. an outer conductor of a coaxial line concentricwith said openings, communicating with and outwardly extending from thewaveguide, said outer conductor being spacially oriented in such amanner that its central axis is coincident with the center of both ofsaid opening; d. a center conductor of said coaxial line concentric withsaid outer conductor, said center conductor comprising an electricalconductor having a curable coating; and e. means for suppressingmicrowave radiation transmitted, via the waveguide, to a centerconductor of the coaxial line wherein said center conductor is saidelectrical conductor whose covering is to be cured.
 2. The apparatus ofclaim 1 wherein the waveguide is a rectangular waveguide.
 3. Theapparatus of claim 1 wherein the waveguide is a circular waveguide. 4.The apparatus of claim 1 wherein the microwave generator operates at afrequency of 2,450 MHz.
 5. The apparatus of claim 1 wherein thesuppressing means comprises at least three baffles in series.
 6. Theapparatus of claim 1 wherein the suppressing means comprises at leastthree resonating cavities in series.
 7. The apparatus of claim 1 whereinthe suppressing means is an alternating combination of baffles andresonating cavities comprising at least one baffle and at least tworesonating cavities.